This invention relates to exothermic anti-piping compositions for use in the casting of molten metal to form ingots or castings. During such operations, just after pouring, it is necessary to maintain the upper portion of the molten metal at as nearly the same temperature as the molten metal in the lower layers as possible. This prevents the forming of undesirable pipe on the surface of the metal by keeping the upper portion of the metal in the molten style by preventing loss of heat therefrom.
One of the ways to prevent such heat loss is by the placing of a powder, granular, or other particulate composition on the exposed molten metal surface, which composition ignites due to the temperature of the molten metal. The composition burns and expands to leave a layer of a heat-insulating residue. This type of composition is referred to as an exothermic anti-piping composition.
Anti-piping or hot topping compositions have been used for several years, and generally include as the basic expanding media acid treated graphite flakes. An example of a hot topping composition utilizing acid-treated graphite is discussed in the Osborn et al U.S. Pat. No. 3,308,514, which utilizes 100% acid-treated graphite flakes as the anti-piping composition. The hot topping taught by the Osborne et al patent, however, is not exothermic, i.e. does not ignite itself and burn, returning heat to the ingot.
Compositions in which the amount of expensive acid-treated graphite has been reduced and replaced by another type of refractory heat-insulating material, and which are exothermic, are discussed in the Rumbold et al U.S. Pat. Nos. 3,804,642 and 3,811,898. While the amount of acid-treated graphite flakes has been reduced, it remains the primary expanding media in such hot topping compositions. In this respect the Rumbold et al U.S. Pat. No. 3,804,642 suggests using 1-50% by weight (preferably 3-20%) of acid-treated graphite and 10-50% by weight of an exothermic component such as aluminum and the like combined with a refractory heat-insulating material.